This invention relates generally to tractor-pulled trailers, and more particularly to a friction relieving device for sliding tandem trailers.
In all states of the United States, there are various weight limits for trucks. Weight limits are defined on a per axle basis. With certain trailers, such as but not limited to flat bed trailers, the position of the load on the trailer may be too far forward of the rear wheels and thus too close to the wheels of the truck cab. In such a circumstance, the per axle load limit may be exceeded by the wheels of the cab. Alternatively, the load may be too far rearward on the trailer thus imposing an undue load on the trailer wheels as opposed to the wheels of the cab. With a sliding tandem trailer, the rear wheel assembly of the trailer may be moved forward or aft with respect to the bed of the trailer as needed so as to adjust the load distribution and insure that the load limits are not exceeded.
With prior art sliding tandem trailers, the movement of the tandem wheel assembly forward or aft with respect to the trailer was difficult, particularly when the truck was loaded. Thus, for example, a given truck may be fully loaded as it leaves its depot. If the loaded truck proceeds to a first weighing station where, for the first time, it is determined that the per axle weight limitation has been exceeded, it is necessary to adjust the load distribution of the loaded truck. It is not feasible, at that point, to unload the truck such that the load distribution may be adjusted in a normal fashion. Accordingly, the load must adjusted in place. To accomplish this technique, with various prior art approaches, the brakes of the trailer are set and the cab is put into a reverse gear. As the rear wheel assembly slides along the bed of the trailer, the cab-trailer wheelbase is shortened. Alternatively, the cab may be placed in a forward gear thus elongating the cab-trailer wheelbase if necessary. Because of the extraordinary friction involved in moving the tandem wheel assembly with respect to the fully loaded trailer, severe loads may be placed upon the cab drive train.
To alleviate this problem, various prior art solutions have been proposed. With one such solution, polyurethane pads have been provided which are situated between the tandem wheel assembly and main frame of the trailer bed. However, these pads have not adequately reduced friction.
In another approach, a retrofit arrangement including rollers situated between the tandem wheel assembly and the main frame of the trailer bed have been provided. One such arrangement is disclosed in U.S. Pat. No. 3,085,816 issued to Tantlinger. The Tantlinger patent is directed to an adjustable suspension system comprising a lift mechanism mounted on the tandem wheel assembly and operable to concurrently disengage a locking means between the tandem wheel assembly and trailer body and to engage a plurality of anti-friction rollers with a complementary track on the trailer body to facilitate relative movement therebetween. When the rollers are disengaged from the track by means of hydraulic pressure, the lock is automatically engaged to preclude relative longitudinal movement between the tandem wheel assembly and trailer body. Similar approaches are disclosed in German Patent No. 1,530,800, in which the moving of a roller from a recessed position to an upper position is used to space a bed from a tandem wheel assembly, and U.S. Pat. No. 3,847,405 issued to Pearce, in which rollers, mounted within housings on two transverse channels, provide a means whereby a slidable frame member may be easily rolled into and out of a main frame member thus telescoping a dump trailer.
One characteristic problem of the above described friction relieving devices, however, is that their performance may often be unsatisfactory in that rust, ice, dirt and the like tend to impede adjustment of the tandem wheel assembly with respect to the trailer body. Because of their open nature, such devices are typically exposed to road hazards which may become lodged in areas of translation thereby increasing the friction involved in moving the tandem wheel assembly with respect to the fully loaded trailer. It is desirable, therefore, to provide a friction relieving device for sliding tandem trailers which is self-contained and which minimizes the exposure of its moving elements to hazards such as rust, ice, and dirt.
Another major problem with prior art devices utilizing rollers as friction relieving devices is that the rollers often develop flat spots caused by the pressure of the loaded trailer bed upon the rollers. Not only must the rollers be capable of moving into and out of engagement with the trailer bed in order to adjust the cab-trailer wheelbase, but also they must be capable of maintaining a substantially cylindrical shape in order to minimize the friction caused by the weight of the fully loaded trailer upon the rollers during adjustment. Typical rollers used in prior art approaches develop flat spots, either across the entire face of a roller as caused by leaving the roller in its engaged position while supporting the weight of the fully loaded trailer, or localized flat spots caused by unequal pressure from the fully loaded trailer due to distorted frame members upon which the rollers translate. That is, the unequal pressures caused by distorted frame members force the bearing surfaces used in such devices to deteriorate, thereby causing the rollers to seize up and subsequently develop flat spots.
Every beam must deflect under the load which is applied to it and it will therefore be distorted into a curved or bent shape. Material on the concave or compression face of a bent beam will be shortened or strained in compression, while material on the convex or tension face will be lengthened or strained in tension. It has been found advantageous in the past to design beams for use as fixed frame members with highly asymmetrical cross-sections. That is, in order to provide a fixed frame member which is capable of withstanding repeated distortions due to load distribution and over-the-road hazards such as bumps and potholes, the beams used as fixed frame members often have an interconnecting web that is offset, or away from the centerline, relative to its upper and lower flanges. Such offset webs, however, also cause distortions in the lower flanges when under load.
The present invention is directed to a means for reducing the friction in such sliding tandem trailers which does not suffer from the above described defects. Accordingly, it is a general object of the present invention to provide a device for sliding tandem trailers which minimizes the friction between the trailer bed and its tandem wheel assembly during adjustment. More specifically, it is an object of this invention to provide a friction relieving device that is especially suitable for adjusting the cab-trailer wheelbase of a fully loaded trailer.
Another object of the invention is to provide an enclosed truck bed lift assembly for adjusting the cab-trailer wheelbase of a sliding tandem trailer, in which the moving parts of the assembly are not exposed to the degrading effects of rust, ice, and dirt.
Yet another object of the invention is to provide a bed lift device which is lightweight, easy to manufacture and install, and which provides both rolling and pivoting action to overcome problems of lower flange distortions caused by offset webs under load.